Clad optical fibers are commonly used to transport radiation from a laser and deliver the radiation at a location remote from the laser where the radiation is required. The cladding of such fibers is typically protected by a polymer sleeve or jacket that covers the cladding of the fiber along the entire length thereof. The fiber and jacket are incorporated in a fiber assembly in which the jacketed fiber is enclosed in a flexible armored sheath having a diameter several times greater than the jacketed clad fiber.
The design of transport fiber arrangements for CW radiation is relatively straightforward. The design of transport fiber arrangements for radiation from high-power Q-switched lasers that deliver laser radiation in pulses, however, can be challenging and becomes increasingly difficult with increasing peak power of the laser radiation pulses. One principle problem that must be overcome is surface damage at the input face of the optical fiber. Another principle problem that must be overcome is heating and damage heating and damage of the fiber protective polymer jacket (see, U.S. Pat. No. 4,678,273). This occurs as a result of some input radiation being directed into the fiber cladding and subsequently escaping from the cladding to be absorbed in the material of the polymer jacket in contact with the cladding. Typically the percentage of input radiation that enters the cladding is less than about 5% of the total. In the case of a laser with 1000 Watts (W) average power output, however, this would potentially result in 50 Watts of radiation being absorbed in the fiber jacket.
One method of reducing surface damage is to contact an elongated glass block to the input face of the optical fiber. The output beam from the laser is focused through the glass block onto the interface between the block and the optical fiber. The input face of the glass block is typically furnished with an anti-reflection coating optimized for the wavelength of the laser radiation. The length of the glass block is selected such that the laser beam at the input surface of the block has a diameter large enough (consistent with the peak pulse power of the radiation) that the peak intensity of radiation is below the damage threshold of the input face of the glass block or any coating thereon. Such an arrangement is described in U.S. Pat. No. 5,619,602. This and similar arrangements are relatively straightforward to implement and do not add significantly to the cost or complexity of the transport optical fiber.
An approach that has been suggested for dealing with damage to the protective polymer jacket of a transport fiber is to encourage radiation in the cladding to leak out of the cladding and be absorbed by a light absorber incorporated in the fiber assembly. The light absorber is water cooled to remove heat generated by the absorbed radiation. This approach is described in U.S. Pat. No. 6,167,177. While the approach is apparently effective, water channels must be provided around the input end of the fiber assembly which considerably increases the complexity of the fiber assembly design. There is a need for a method for removing cladding-mode radiation that does not require water-cooling the input end of the fiber assembly.